[modules] Refactor code from ASTReader::makeModuleVisible() into a new function,
Module::getExportedModules() so it can be reused.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175548 91177308-0d34-0410-b5e6-96231b3b80d8
[modules] Const'ify some functions of ModuleMap.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175552 91177308-0d34-0410-b5e6-96231b3b80d8